On Mon, May 10, 2010 at 5:51 AM, Milind Patil wrote:

For a function

f ::  a -> m b f = undefined

I am having trouble understanding how the type of

(>>= f)

is

(>>= f) :: m a -> m b

where, by definition, type of (>>=) is

(>>=) :: (Monad m) => m a -> (a -> m b) -> m b

I do not see how (>>= f) even unifies.

I mean if I code a function with the same type as (>>=) ie.

tt :: (Monad m) => m a -> (a -> m b) -> m b tt = undefined

type of (tt f) does not infer to the same type as (>>= f), from ghc ...

(tt f) :: (Monad ((->) b)) => (m a -> b -> b1) -> b -> b1

There seems to something special about (>>=) apart from its type. And whats (Monad ((->) b))? I am new to Haskell and I may have gaps in my understanding of type inference in Haskell.

It's because >>= is a binary operator. When you partially apply a binary operator, you get a "section" which applies one of the two arguments. Specifically, you have: (>>=) = \m f -> m >>= f (m >>=) = \f -> m >>= f (>>= f) = \m -> m >>= f There's more in the Haskell tutorial (section 3.2.1) http://www.haskell.org/tutorial/functions.html Or you can check the Haskell Report, section 3.5: http://www.haskell.org/onlinereport/exps.html#sections -- Dave Menendez http://www.eyrie.org/~zednenem/

On May 10, 2010, at 05:51 , Milind Patil wrote:

There seems to something special about (>>=) apart from its type. And whats (Monad ((->) b))? I am new to Haskell and I may have gaps in my understanding of type inference in Haskell.

Everyone else having answered the first question, I'll take this one: ((->) r) is the monad of functions with a single argument (and therefore all functions, as Haskell curries all functions: that is, "f a b c" is a function which takes an a and returns a function that takes a b, which in turn produces a function that takes a c and produces the final result). It's also known as the Reader monad. The syntax is the function form of the section (r ->) (which I think is otherwise illegal, since -> is actually syntax rather than an operator); understand this as a partial application of (r -> x), a function which takes an r and produces an x. -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allbery@kf8nh.com system administrator [openafs,heimdal,too many hats] allbery@ece.cmu.edu electrical and computer engineering, carnegie mellon university KF8NH